CAD 516 Ground Proximity Warning System

5.2.6 Some predictive ground proximity warning systems require barometric
altitude referenced to Mean Sea Level (ie QNH) for correct operation of
look-ahead alerting and terrain display. Barometric altitude referenced to
airport elevation (QFE) or an error made in altimeter setting therefore, may
cause nuisance alerts and incorrect terrain display.
5.3 Other Limitations
5.3.1 The terrain display of predictive GPWS is intended to serve only as a
situational awareness tool and may not provide the accuracy and/or fidelity
upon which to solely base terrain avoidance maneouvering decisions.
Navigation should not, therefore, be predicated upon the use of the database
terrain display.
5.3.2 Predictive GPWS database, displays and alerting algorithms may not account
for man-made obstructions.
6. OPERATIONS MANUALS AND CHECK LISTS
6.1 Operations Manuals should contain, in their introduction to GPWS, information
similar to that noted in paragraph 3 above. The fact that a GPWS alert or warning
tells the crew that the flight path of the aircraft may not be as they suppose it to be,
should be emphasised.
6.2 Technical details of the system should contain brief descriptions of:
6.2.1 The input sources, with particular reference to the Radio Altimeter (and
Barometric Altimeter in the case of predictive systems);
Issue 1 7 June 2001